The present invention relates to a method of dynamically adapting the capacity of a contention transmission channel, shared on an up-path by and from a plurality of terminals to a network connection gateway.
The present invention also relates to a method of transmitting data packets or fragments of packets by a plurality of terminals to a network connection gateway, the transmission method using the said method of dynamically adapting the capacity of the contention transmission channel.
The present invention further relates to a system for implementing the method of dynamically adapting the capacity of the contention transmission channel and to a system for implementing the method of transmitting data packets and fragments of packets.
Generally, the invention is applicable to any communications system requiring a contention transmission channel on an uplink whose traffic is sporadic, dense and non-predictable, and able to use for example transparent or regenerative satellites and/or terrestrial wireless connections, or even cable-based connections.
Various contention access schemes are known, including the time-segmentation or slotted ALOHA protocol and its derivatives combining the capture effect CE and/or the effect of using (temporal or frequency) diversity and access conflict resolution CRD (Contention Resolution Diversity).
All these protocols are random protocols in which each user terminal accesses the transmission resources independently in relation to the other users. For each packet transmitted, the user waits for an acknowledgement of receipt from the recipient. If he does not receive it, he retransmits the same data with a random delay and this mechanism is repeated until receipt of an acknowledgement of receipt or until a maximum number of attempts have been made.
In order to avoid the congestion of the transmission channel in case of channel overload, it is known to implement a congestion control mechanism in the form of an algorithm.
The European patent published under the number EP 1 686 746 B1 describes a first congestion control algorithm within the framework of a slotted ALOHA protocol with diversity and with conflict resolution CRDSA (“Contention Resolution Diversity Slotted Aloha”). This algorithm uses information about the network loading. The way in which this network loading is evaluated is however not specified in the document. Information about the network loading originating from a central entity is regularly transmitted to the various terminals. If this loading exceeds a threshold, each terminal will increase, according to a decreasing probability and independently, the transmission delay of the fragments that it must send by an additional logical frame. If the loading decreases below the threshold, the terminal will also reduce, according to a given probability, the sending delay of its fragments by a logical frame. Each terminal will therefore wait for a different number of logical frames as a function of the inherent evolution of its congestion window, knowing that it will only be possible for a fragment and its replicas to be transmitted on a single logical frame. Logical frame is understood to mean a time interval of fixed duration defined by the telecommunications standard or system employed to communicate and which constitutes the time unit making it possible to fix a reference for the senders and for the receivers for the transmission and reception of data packets or fragments of these packets. A logical frame can be composed of a given number of time slots or elementary physical frames. In particular, in the case of contention access schemes which, for each fragment of useful data to be sent, generate one or more redundant fragments, such as for example the CRDSA scheme, the set of fragments (useful and redundant) is transmitted in a logical frame.
The advocated approach relating to congestion control in the European patent published under the number EP 1 686 746 B1, is a reactive approach which reacts to the overloading of the transmission channel by using a notion of loading threshold and which is therefore not a preventive approach. A non-negligible number of collisions may thus occur temporarily when the threshold is crossed and it could be that, as the reaction time for this overload is too large, this causes retransmissions of messages or message fragments causing an increase in the message transmission lag. This approach also involves a congestion and channel access control policy which is distinct between terminals and which is not managed in a centralized manner, and this may potentially penalize certain terminals and favour others. This approach may potentially cause an inequity between the user terminals and makes it difficult to apply distinct levels of quality of services for packets originating from one and the same terminal or from different terminals. These schemes also exhibit the drawback of artificially increasing the loading of the network due to the replicas generated systematically for each packet to be sent, thus making it more difficult to evaluate the actual loading, stated otherwise the loading related to the useful data.
In order to remedy the aforementioned drawbacks, document EP 2 787 702 A1 proposes a preventive approach and describes a congestion control algorithm for contention access network making it possible to minimize the number of retransmissions. The algorithm comprises a first step in which the number of terminals is continually evaluated during sending and a second step in the course of which is defined a sliding window of sending from terminals, depending on the estimation of the number of terminals and being aimed at regulating and spreading their transmissions. Thus, the rate of initial collisions (or initial unresolved collisions for schemes sending several data packets for a useful packet) between packets sent by several senders is decreased, the number of necessary retransmissions becomes substantially zero and the lag in transmitting a packet to its destination is substantially shortened. In this manner, preventive spreading makes it possible to keep the loading below a chosen operating point so as to avoid the collapse of the network. Furthermore, this second algorithm is executed in a centralized manner (no need for teledetection capacity for the senders), so all the active terminals use, at a given instant, the same congestion window, thereby allowing precise control of the congestion level and making it possible to ensure equity between terminals.
The two aforementioned documents are limited to the description of a method of controlling congestion of a slotted contention channel and presuppose a capacity of the contention channel which is fixed and unmodifiable. The problem of making sufficient channel capacity available, suitable for the traffic need of terminals, is neither described nor mentioned in these documents.
To date, the known transmission systems and methods which implement a slotted (synchronous) or non-slotted (asynchronous) contention transmission channel use a band of frequencies or more generally a set of communication resources that is preconfigured or modifiable at the very most through time scheduling, so as to offer the contention channel an adapted capacity suitable for smooth and predictable traffic of the terminals.
When the actual traffic of the terminals becomes dense, very sporadic and non-predictive, current systems and methods give rise to the following major deficiencies.
Firstly, a loss of capacity is created by over-dimensioning the resources allocated to the contention channel regardless of its type (slotted (SA) or non-slotted ALOHA channel, or higher-performance variants such as CRDSA). On average, this overcapacity is much greater than the instantaneous need of actual traffic to absorb non-predictable traffic spikes.
Thereafter, the definition of a dimensioning of the contention channel for a mean traffic profile poses a problem, related to unforeseeable but statistically inevitable congestion spikes on the channel, of the occurrence of very high access lags and therefore a reduction in the overall performance of the system, or indeed a collapse of the network.
Finally, modifying the capacity of the contention channel by time scheduling so as to follow a mean “profile” becomes useless when unforeseeable events (natural catastrophes for example) arise, since scheduling is a “model” which, on average, follows the traffic need of users, but does not correspond to the instantaneous and unforeseeable need in terms of communication resources.
Consequently, the definition of a “mean” capacity to be provided for in respect of the shared contention channel appears to be a difficult or indeed impossible task. At the user level, the network becomes unavailable with an increase in the access time and/or inefficient preventive congestion mechanisms on rebooting the network. At the system level, availability performance decreases.
The technical problem is to propose a method and a system for dynamically adapting the capacity of a contention transmission channel, shared on an outbound pathway by and from a plurality of terminals to a network connection gateway in order to respond rapidly and suitably to a demand for dense, sporadic and non-predictable traffic whichever variants of the ALOHA SA mode (CE, CRDSA, etc.) or non-slotted mode are used for the contention channel.